@Nellousan You don’t specify which Nakama runtime you use, but based on the function names I’ll assume you want to work with Go.
There’s a function available via the NakamaModule interface called ReadFile, you can use to read a file from disk in InitModule. This file might be a JSON file you can then parse and use however you need in your server-side game logic. 
Collections could also work, and they would be fine for this use case, but I wouldn’t recommend that approach unless you need the data to be reloadable without a Nakama server restart. This has its own layer of complexity, so stick with the file read at startup if you can.